Why is it better than poe.trade?
Because it has much better support for buy-orders as well as bulk-sales, and allows you to more easily sort by the time of the listing.
It is supremely annoying having to pm a dozen people to get someone who actually replies to your trade request, and equally annoying to tp out of your map to do a trade.
It's weird how people constantly complain about this, yet I personally never have this issue. And it's because I actually value my time highly enough to pay a "premium" for something I need. When there's 100 listings at 1c each, I just go and message someone who has it up for 2c instead. Because 9/10 times, that person with the higher price still has the item. Stop trying to get the cheapest deal, in the time it takes you to get the item you want for the lowest possible price, you could've farmed the difference 10 times over.
Now you could argue that with an AH you wouldn't have to do that, but guess what, unless you're actively sniping new auctions you would still pay the higher price anyway. Just because some exalts are listed at 160c right now, doesn't mean that you're entitled to only having to pay that much. The real value is the price at which you actually get the item, and AH or not, that would be closer to 165-170c right now.
This argument is also where buy-orders come in. I delved pretty heavily in blight, got to ~1k depth within the first two weeks and I didn't message someone for single sulphite scarabs at all. I bought in bulk, and when no bulk-sales were available I just set a buy-order and people came to me with their scarabs instead. Could I have tried to message dozens of people who had their rusted scarabs listed for 20c? Sure. Instead I set up a buy order for 25c and lo and behold within hours I had stocked up over 100 scarabs. It was worth paying extra, because that way I had almost no time-expense doing the actual trading.
Same applies to leaving your map to trade. Don't do that. If someone buys a shitty unique for 2c, leaving your map early is just gonna make you lose money. If the guy wants the item, he's not gonna mind waiting for you to finish your zana, lab or delve, let alone finishing just a regular map.
The only thing an automated trading system would do is remove the price-fixed listings, which you can easily blacklist, while creating a much bigger opportunity for some of the most wealthy players to just corner an entire section of the market - something which is much much worse than having to scroll down a few times and paying slightly more than the lowest listing price.